The present invention relates generally to heat exchanger apparatus, and more particularly relates to combustion heat exchanger apparatus used in fuel-fired forced air heating furnaces.
In a fuel-fired forced air furnace, air returned from the conditioned space served by the furnace is flowed across a hollow combustion heat exchanger disposed within the furnace housing while the heat exchanger receives a throughflow of a burning fuel/air mixture discharged from the furnace burner section. As the air externally traverses the heat exchanger, combustion heat is transferred to the air from the outer surface of the heat exchanger. The heated air is then discharged from the furnace for resupply to the conditioned space.
Furnace heat exchangers of the type described are required to simultaneously and efficiently perform two functions. First, they must interiorly provide for essentially complete combustion of the burning fuel/air mixture received thereby with an acceptably low level of exhaust emissions. Second, they must also provide for efficient combustion heat transfer to the supply air being flowed through the furnace.
In conventional furnace heat exchanger design, meeting each of these two goals has proven to be a challenge since the heat exchanger geometry needed to provide a high degree of combustion efficiency is often markedly different than that required to provide a high degree of combustion product-to-air heat transfer efficiency. In other words, a good heat exchanger geometry for combustion may be much less desirable for its simultaneous heat transfer task, and vice versa.
One approach used to overcome these competing geometric needs has been to fashion the heat exchanger in two sections--one combustion section and one heat transfer section--of different geometric configurations. An example of this design approach may be seen in U.S. Pat. No. 4,974,579 to Shellenberger et al in which a furnace heat exchanger is illustrated as having a combustion section formed from a spaced series of relatively large diameter metal tubes, and a heat transfer section formed from a spaced series of relatively small diameter metal tubes. The large diameter combustion tubes provide the relatively large combustion volume necessary to provide for high combustion efficiency, low CO emission and low combustion noise, while the smaller diameter heat transfer tubes provide enhanced heat transfer capacity due to their relatively higher surface area and heat transfer coefficient.
A limitation in this approach is the need for an intermediate portion to connect the differently configured combustion and heat transfer sections of the heat exchanger--for example, a metal collector box as illustrated in the aforementioned U.S. Pat. No. 4,974,579. The use of these three separate heat exchanger sections of course adds to the expense of manufacturing and fabricating the heat exchanger. Additionally, because of the high temperature of the combustion gases exiting the large tube combustion section of the heat exchanger the intermediate section thereof needs to be of a rugged metal construction to withstand the high thermal stress levels experienced.
In an attempt to reduce the cost of forming a tubular metal furnace heat exchanger, a serpentined array of metal heat exchanger tubes, of a single size, has been investigated. While this approach is more economical as to manufacturing and fabrication costs, it entails a somewhat undesirable tradeoff between combustion and heat transfer efficiency as discussed above. For example, by using relatively large diameter tubes to form the entire heat exchanger, high combustion efficiency is achieved. However, this relatively large diameter tube size is not particularly efficient from a heat transfer standpoint. Accordingly, from an overall tubing length standpoint it was necessary to oversize the heat exchanger to provide it with sufficient heat transfer capacity. This made the heat exchanger voluminous and caused excessive pressure losses both in the flue gas side and the air side of the heat exchanger.
In view of the foregoing it can readily be seen that a need exists for an improved furnace heat exchanger construction that provides a high degree of both combustion and heat transfer efficiency. It is accordingly an object of the present invention to provide such an improved furnace heat exchanger construction.